1. Technical Field
This invention relates to fabrication of holographic devices. More particularly, this invention relates to fabrication of holographic devices employing multiple holograms.
2. Background of the Invention
In recent years, there has been an increased use of lasers, and other light sources, in commercial and military applications. Some of the more recent applications of lasers include, without limitation, communication systems, imaging systems, target designators, rangefinders, displays, gyros, and holography.
Radiation emitted by lasers, if not shielded against, has the potential to damage human eyes, as well as light-sensitive devices that employ lens to focus light, such as photo-imaging devices and light sensors. Thus, it is desirable to produce a light-protective device, meaning a device capable of protecting eyes and other light-sensitive devices from potentially injurious light radiation, such as may be encountered with lasers. Various methods and devices for diffracting or filtering light, to protect against light radiation, are known in the art. In one method, an absorptive dye is incorporated into a lens element situated in the optical path of the eyes or a light-sensitive device. The dye acts as a filter and absorbs some undesired radiation. The use of absorptive dyes, however, has some disadvantages. For instance, the dyes used often are inherently wide band absorbing. The resultant wide band absorption has an undesirable effect of rejecting certain colors of an image viewed through the dye. This results in an image that is unsuitable for many applications.
In a second method, devices utilizing conventional multi-layered coatings may be applied to a substrate to protect objects from undesired light radiation. The multiple layers are chosen to diffract or filter various wavelengths of light depending on the coating of the particular layers. Unfortunately, many of these coatings are difficult to apply to complicated substrate shapes. As a result, limitations are placed on the useful applications of these coatings where complex-shaped substrates are involved.
The use of holograms, including transmission and reflection holograms, to diffract light radiation has been proposed as a solution to the problem of protecting against undesired radiation. The use of single-layer holograms, having multiple holographic images on a single holographic film, to diffract various wavelengths of incident radiation has been suggested to serve this purpose. The presence of multiple exposures within a single layer, however, tends to result in a lowering of index modulation. Consequently, image brightness is lowered, the result of which might lead to cross-talk between images, and more generally, to less efficient devices.
It also has been proposed to use laser reflective devices that employ a plurality of conterminous holograms, disposed on a substrate or adjacent substrates, to optimize reflection of undesired light radiation. See U.S. Pat. Nos. 4,601,533 and 4,637,678. These devices utilize multiple holograms, each having a predetermined fringe spacing, to reflect a given wavelength. The multiple holograms are bonded together with an additional adhesive layer, such as an epoxy. The use of multiple holograms has the advantage over single holograms in that it gives a wider angular coverage of wave-length reflection due to the ability to vary fringe spacing in the various layers. Thus the use of multiple layers improves reflectivity by the ability of each layer to reflect a different wavelength.
A disadvantage to the holographic light-protective devices disclosed in the above patents is that the multiple holographic layers are bonded to each other by the use of the additional adhesive layer. The additional adhesive layer tends to add undesirable thickness to the resultant device. Consequently, prior art devices are impractical for many applications where device thickness is an important consideration. Additionally, the presence of an adhesive layer renders difficult the registration of wave-fronts, and also of the holograms themselves. Therefore, efficient fabrication of these type of multi-layer holograms is inhibited by the presence of the adhesive layer.
The use of multi-color holography is described in connection with using a film of multiple color sensitivities in U.S. Pat. No. 3,442,566.
In short, it is desirable to produce on a variety of substrate surfaces a device employing multiple holograms which exhibits excellent light-protective characteristics and has a relatively thin cross-sectional thickness. The literature, however, fails to provide a teaching as to how to overcome one or more of the problems discussed above.